As a binder generally and widely used for a magnetic recording medium, thermoplastic resins such as a vinyl chloride and vinyl acetate type resin, a vinyl chloride and vinylidene chloride type resin, a cellulose type resin, an acetal resin, an urethane resin, an acrylonitrile-butadiene resin and so on are used alone or in combination. However, use of the above binders is disadvantageous in that (1) the wear resistance of the magnetic layer is poor and (2) the running paths of the magnetic tape become stained.
Further, it is known that thermosetting resins such as a melamine resin, urea resin and the like are used as a binder and that a binder which is cross-linked by a chemical reaction such as a compound having an epoxy ring or an isocyanate compound is added to the above-described thermoplastic resins and is used as a binder. However, the use of a cross-linkable binder is disadvantageous in that (1) the storage stability of a solution wherein magnetic particles are dispersed is poor and accordingly, a homogeneous physical properties of the magnetic coating composition as well as a homogeneous quality of the magnetic recording medium cannot be obtained, and (2) a step of heat treatment for hardening a coated film after drying is indispensable and it takes a long time for the heat treatment.
In order to overcome the above-described disadvantages, a method for preparing a magnetic recording medium which comprises using an oligomer and an monomer of acrylic ester type as a binder and hardening the binder by radiation exposure after drying is known, as disclosed in Japanese Patent Publication No. 12423/1972, Japanese patent application (OPI) No. 13639/1972, 15104/1972, 77433/1975 and 25231/1981. However, in accordance with the above-described method, a magnetic recording medium having high electromagnetic properties and durability cannot be obtained.
Recently, high image quality of a magnetic recording medium has been required. For the above purpose, it is necessary that a video head and an audio head should be closely contacted with the surface of a magnetic layer and that not only the surface smoothness of the magnetic recording medium should be improved but also the dispersibility of the ferromagnetic fine particles should be greatly improved. However, as the surface of the magnetic layer becomes smoother, the friction of the running system of the video tape recorder becomes higher. Therefore, the running tension becomes high. As a result, excellent running durability of the magnetic recording medium under harsh conditions has been required more and more. For the above reasons, in accordance with the conventional method for preparing a magnetic recording medium, a magnetic recording medium having an adequate surface smoothness of the magnetic layer, good dispersibility of the ferromagnetic fine particles and running durability cannot be obtained.